Gas supplementation method of excimer laser apparatus

ABSTRACT

In an excimer laser apparatus in which halogen gas, rare gas and buffer gas are fed into the laser chamber, before laser oscillation, the oscillation stop time is calculated, and, if the calculated oscillation stop time exceeds a prescribed time, the calculated oscillation stop time is used to calculate a feeding amount of mixed gas comprising rare gas or buffer gas, and the mixed gas is fed, prior to laser oscillation, in the calculated feeding amount; stable laser output is thereby obtained from the initial period of laser oscillation.

TECHNICAL FIELD

The present invention relates to an excimer laser apparatus employed forexample in light sources for reduced-size projection exposure,microprocessing of materials, or surface improvement of materials; inparticular, it relates to improvements for enabling the laser tooscillate in stable fashion over a long period.

BACKGROUND ART

Conventionally, when an excimer laser apparatus employing halogen gas isoperated, the halogen gas is consumed, during the process of operation,by evaporation of the electrode materials and chemical reaction with theconstituent material of the laser chamber. Conventionally therefore,control was performed as follows in order to compensate for the loweringof laser output produced by consumption of halogen gas.

Specifically, the laser output is obtained by passing through adischarge space electrical energy for laser excitation that wasaccumulated on a capacitor, the discharge being effected in laser mediumgas; if the charging voltage of this capacitor is raised, laser outputis increased. Conventionally therefore laser output was stabilized bydetecting the laser output and controlling the value of the chargingvoltage in accordance with the results of this detection. Such controlis called "power lock control" and this charging voltage willhereinbelow be called the "power lock voltage".

However, even with such control, if operation is continued for a longtime, the efficiency of oscillation is lowered by consumption of halogengas, with the result that the prescribed output cannot be maintainedunless the charging voltage (power lock voltage) is progressivelyraised.

Conventionally therefore arrangements were made to attempt to cope withthis consumption of halogen gas by arranging for supplementation with afixed quantity of halogen gas when the charging voltage increased abovesome prescribed voltage.

Such a method of halogen gas supplementation according to the prior artwill be described with reference to FIG. 29 to FIG. 31.

In more detail, FIG. 29 shows structural parts pertaining to gassupplementation of a typical fluorine-based excimer laser apparatus; inthis case, there are provided, as a gas feed cylinder, a cylinder 41that is charged with a halogen gas (F2, HCl etc) diluted with a buffergas (Ne or He etc), a cylinder 42 charged with a diluent gas such as Kr,and a cylinder 43 charged with a buffer gas such as Ne or He; wheneffecting gas feed on start-up, gas feed to laser chamber 47 is effectedby open/shut control of on/off valves 44, 45, 46, and, whensupplementing the halogen gas after operation, gas feed is effectedthrough on/off valves 48, 49 and "subtank" 50.

In more detail, when introducing new gas into laser chamber 47 beforestart-up, first of all, the old gas in laser chamber 47 was dischargedby means of on/off valve 51 and vacuum pump 52.

Next, Kr gas is introduced into laser chamber 47 to a pressure of 40torr from cylinder 42 through on/off valve 45; next, F2 gas diluted byNe gas is introduced to a pressure of 80 torr from cylinder 41 throughon/off valve 44; finally Ne gas is introduced from cylinder 43 throughon/off valve 46 to make the overall pressure in laser chamber 47 2500torr.

By such gas feed control, the gas composition within laser chamber 47 inthis apparatus becomes F2:Kr: Ne=4:40:2456 (torr), i.e.F2:Kr:Ne=0.16:1.60:98.24 (%) in terms of concentration ratios.

Thus, when laser chamber 47 is charged with new gas, gas supplementationcontrol is performed by the procedure shown in the flow chart of FIG. 30during subsequent laser operation.

First of all, before operating the excimer laser apparatus, the targetlaser output Ec, optimum control charging voltage range Vm (Vmin toVmax), the increase/decrease charging voltage ΔV when control isexercised once, and the one-time supplementation gas amount ΔG are setbeforehand (step 500).

When operation is then started, controller 55 gets the laser output Edetected by laser output monitor 53 and the charging voltage V detectedby charging voltage detector 54 (step 510). Controller 55 comparesdetected laser output E with the target laser output Ec (step 520); ifE<Ec, it raises the detected charging voltage V by the minute voltage ΔVand makes this the instruction charging voltage Va (step 530); if E=Ec,it leaves the detected charging voltage V unaltered and takes this asinstruction charging voltage Va (step 540); if E>Ec, it lowers thedetected charging voltage V by the minute voltage ΔV and takes this asthe instruction charging voltage Va (step 550).

Furthermore, controller 55 compares instruction charging voltage Va withthe maximum value Vmax of the maximum control charging voltage range Vm(step 560); if Va<Vmax, it returns again to step 510, to perform controlof instruction voltage Va. However, if Va>Vmax, supplementation of F2gas containing Ne gas from cylinder 41 in a prescribed amount ΔG iseffected into laser chamber 47 (step 570), and some of the gas isdischarged (step 580) so as to maintain the prescribed overall pressureof the gas in laser chamber 47.

FIGS. 31(a) to 31(d) show respectively time charts in respect of laseroutput E, instruction charging voltage Va, halogen gas (F2)concentration, and diluent gas (Kr) concentration resulting from theabove control; the laser shot number is taken along the time axis. InFIG. 31(b), the time points t1 to t6 at which the charging voltageinstruction Va to the capacitor suddenly drops correspond to the timeswhen supplementation of halogen gas is effected.

However, with the prior art technique, gas cylinder 41 that is used forhalogen gas supplementation does not contain a diluent gas constituent.Furthermore, with this prior art technique, every time gassupplementation occurs, control is exercised such as to maintain theoverall pressure constant (step 580 of FIG. 30) by discharging some ofthe gas in laser chamber 47, so, every time gas supplementation isperformed, the amount of diluent gas (Kr) is gradually decreased asshown in FIG. 31(d): a simple calculation shows that 20% of the Kr gasis eliminated by ten gas supplementations. In other words, this meansthat, as shown in FIG. 31(c), as gas supplementation of the halogen gas(F2) goes on, there will gradually be over-supplementation. Also, evenif the one-time supplementation amount ΔG is set sufficiently small sothat over-supplementation does not occur, as shown in FIG. 31(c), thesupplementation intervals then gradually decrease with the result that,in the end, the gas balance is destroyed. That is, with the prior arttechnique described above, every time supplementation of halogen gas iseffected, the optimum compositional balance of the mixed gas in laserchamber 47 is lost, with the result that it becomes impossible tomaintain a fixed laser output, no matter how the charging voltage to thecapacitor is controlled.

Furthermore, with the prior art, there was the problem that, sincehalogen gas was supplemented in fixed amount every time in response to acomparison of the charging voltage but irrespective of the optimumcompositional balance of the gas, it was difficult to maintain theoptimum compositional balance, and the gas balance could easily be lostby external disturbances.

Furthermore, with the prior art, regarding gas exchange, this was alwaysperformed with the same gas composition, considered as optimum. However,with use of the laser for a long period of time, impurities and/or dustaccumulate in the chamber so that there is a progressive fall-off inlaser output. For this reason, even though gas exchange is performed,the charging voltage needed to obtain a prescribed laser outputgradually becomes higher.

Furthermore, although, in some examples of the prior art, control offeeding of halogen gas is performed, this merely consists of the type ofcontrol in which a fixed quantity of halogen gas is intermittently fed,so there was the problem that the laser output was lacking in stability.

Also, with the prior art, there was no automatic notification of thetime for changing the gas or the time when maintenance was due, so theoperator had to make a decision about these from the dirtiness of thewindow or condition of deterioration of the gas, or the life of thelaser chamber etc: this was difficult for an operator of limitedexperience.

Moreover, when laser oscillation is stopped, the gas in the laserchamber continues natural deterioration with lapsed time. However, inthe prior art, no measures were taken regarding laser stoppage, so whenlaser oscillation was commenced, there was the problem that laser outputwas unstable. This was particularly marked if the laser was stopped fora long period.

With the foregoing in view, it is an object of the present invention toprovide a method of gas supplementation for an excimer laser apparatuswhereby loss of the optimum composition balance of the gas becomesunlikely even when gas supplementation is repeated many times; whereinthe time point of gas supplementation and the amount of gassupplementation can be determined accurately; wherein stable laseroutput can be obtained even when the laser is stopped for a long aperiod; and wherein the operator is notified automatically andappropriately of the time for gas exchange and the time for maintenance.

DISCLOSURE OF THE INVENTION

In an excimer laser apparatus in which laser oscillation is performedwith feeding of halogen gas, rare gas and buffer gas into a laserchamber, the first invention is characterized in that, for laseroscillation, an oscillation stop time is calculated, and, if thiscalculated oscillation stop time exceeds a prescribed time, thiscalculated oscillation stop time is used to calculate a feeding amountof mixed gas comprising rare gas and buffer gas, and the mixed gas inthe amount of this calculated feeding amount is fed prior to laseroscillation (power correction subroutine 1).

With the first invention, the rare gas and buffer gas are supplementedin an amount corresponding to the oscillation stop time, so laseroscillation is performed after returning the output that had shown anatural deterioration during laser stoppage, to the correct originalcondition.

Since, with the first invention, supplementation of rare gas and buffergas is thus performed in an amount corresponding to the oscillation stoptime prior to laser oscillation, laser oscillation can be performedafter restoring the output drop of the laser due to naturaldeterioration during laser stoppage to its original correct condition; astable laser output can thus be obtained from the initial period oflaser oscillation.

In the second invention, the halogen gas partial pressure within thelaser chamber is detected, and the supplementation amount of halogen gasis determined in accordance with this detected partial pressure value.

With the second invention, the supplementation amount of halogen gas isdetermined in response to the halogen gas partial pressure, which is theparameter that has the most direct relationship with the amount ofdiminution of halogen gas.

Thus, with the second invention, the partial pressure of the halogen gaswithin the laser chamber is detected, and the amount of supplementationof halogen gas is determined in response to the detected partialpressure value, so halogen gas can be supplied accurately in the amountof the diminution of halogen gas.

With the third invention, feeding of mixed gas comprising rare gas andbuffer gas is not performed prior to laser oscillation, but if, afterlaser oscillation, the laser output during this laser oscillationdeparts from the rated laser output, the charging voltage is detectedand an amount of mixed gas comprising rare gas and buffer gas is fedthat is equal to the amount calculated in accordance with the chargingvoltage that was thus detected (power correction subroutine 3).

With the third invention, feeding of rare gas and buffer gas is notperformed prior to laser oscillation. However, immediately after laseroscillation, if the laser output during oscillation exceeds the ratedlaser output, rare gas and buffer gas are fed in amount calculated inaccordance with the detected charging voltage. That is, if the laseroutput exceeds the rated laser output, normally the charging voltagewill have a fairly high value; in this case, the charging voltage valueis immediately lowered by feeding at once a large quantity of rare gasand buffer gas corresponding to this large value of charging voltage.

Thus, with the third invention, the charging voltage is controlled suchthat a desired laser output is obtained immediately after laseroscillation, and the charging voltage when the desired laser output hasbeen obtained is detected, rare gas or buffer gas being fed in an amountcalculated in accordance with the detected charging voltage;consequently, even if the charging voltage is in a condition higher thaninitially, it is possible to feed at once a large quantity of rare gasor buffer gas corresponding to this large charging voltage value, withthe result that the charging voltage value can be brought down at astroke, in comparison with the conventional system of feeding of fixedquantities of gas.

With the fourth invention, if a gas exchange request signal is outputprior to laser oscillation, the initial power lock voltage in thepresently charged gas is detected and this detected value is comparedwith a prescribed threshold value; if the detected value is smaller thanthe threshold value, gas exchange is effected with gas of the samecomposition as on the previous occasion; if the detected value is largerthan the threshold value, the feeding amount of mixed gas comprisingrare gas and buffer gas is altered in accordance with the deviationbetween this detected value and the threshold value, and gas exchange isperformed with a gas composition based on this altered feeding amount(power correction subroutine 1, power correction subroutine 2).

With the fourth invention, in gas exchange, whether or not to change thecomposition of the gas is determined in accordance with the power lockvoltage and, if the gas composition is to be changed, the feeding amountof rare gas and buffer gas is determined in accordance with the powerlock voltage.

Thus, with the fourth invention, if a gas exchange request signal isoutput prior to laser oscillation, the gas composition relating to therare gas and buffer gas is altered in accordance with the initial powerlock voltage in the presently charged gas, so if for example the initialpower lock voltage is high, gas exchange is effected with a gascomposition in which rare gas and buffer gas have been added to the gascomposition used on the previous occasion, so the prescribed laseroutput can be obtained from the comparatively low charging voltage oncommencement of laser oscillation.

With the fifth invention, if the charging voltage after laseroscillation exceeds the prescribed threshold value, a prescribed amountof mixed gas is fed at the time point where the time for whichoscillation has occurred with the charging voltage above threshold valuehas exceeded the prescribed time interval, after feeding of mixed raregas and buffer gas on the previous occasion (Kr/Ne mixed gas feedingsubroutine 1).

With the present invention, if the charging voltage exceeds theprescribed threshold value, feeding of mixed gas is performed inprescribed amount corresponding to the condition in which the chargingvoltage has come to be above the threshold value during the aforesaidtime interval; accurate and precise laser output fixed control canthereby be achieved by feeding of rare gas and buffer gas, even whencontrol is performed by supplying a fixed quantity of gas.

With the sixth invention, if, prior to laser oscillation, a gas exchangerequest signal is output, a window exchange signal or maintenancerequest signal is output in response to the presently charged gasoscillation shot number or presently charged gas charging time or gaspressure within the laser chamber or number of gas charges after windowexchange of the laser chamber (gas exchange request signal outputsubroutine).

With the sixth invention, a window exchange signal or maintenancerequest signal is automatically output in response to oscillation shotnumber of the presently charged gas or charging time of the presentlycharged gas or gas pressure within the laser chamber or number of timesof gas charging after window exchange of the laser chamber.

Thus, with the sixth invention, since a window exchange signal ormaintenance request signal is automatically output in response tooscillation shot number of the presently charged gas or charging time ofthe presently charged gas or gas pressure within the laser chamber ornumber of times of gas charging after exchange of the window of thelaser chamber, the correct time for maintenance or window exchange canbe ascertained by the operator.

The seventh invention is characterized in that feeding of mixed gascomprising rare gas and buffer gas is performed by a mass flowcontroller and charging voltage is detected after laser oscillation, thefeeding flow rate of the mixed gas being altered in accordance with thedifference between this detected charging voltage and the upper limitvalue of the charging voltage (Kr/Ne mixed gas feeding subroutine 3).

Thus, with the seventh invention, precise feeding can be achieved evenin minute amounts by the mass flow rate controller and flow rate controlof the mass flow controller is performed in response to the differencebetween the charging voltage and the upper limit value of this chargingvoltage; precise and accurate laser output fixed control can thereby beachieved by feeding of rare gas and buffer gas.

Thus, with the seventh invention, feeding of mixed gas comprising raregas and buffer gas is performed using a mass flow controller and thecharging voltage for power lock is detected after laser oscillation, thefeeding flow rate of the mixed gas being altered in response to thedifference between this detected charging voltage and the chargingvoltage threshold value; accurate and precise laser output constantcontrol can thereby be achieved by means of rare gas and buffer gasfeeding.

With the eighth invention, the laser shot number since feeding of themixed gas comprising rare gas and buffer gas on the previous occasion iscalculated, and if this shot number exceeds the prescribed thresholdvalue, mixed gas is fed (Kr/Ne mixed gas feeding subroutine 2).

With the eighth invention, feeding of rare gas is performed based on thelaser shot number, which is in direct relationship with the laseroscillation rate.

Thus, with the eighth invention, the laser shot number occurring afterthe feeding of the mixed gas comprising rare gas and buffer gas on theprevious occasion is calculated, and mixed gas feeding is performed ifthis shot number exceeds a prescribed threshold value; feeding of raregas is therefore performed based on the laser shot number, which has adirect relationship with the laser oscillation rate, so accurate andprecise laser output control can be achieved.

With the ninth invention, the oscillation spectral line width isdetected, and the amount of halogen gas to be fed is determined inaccordance with the difference between this detected line width valueand a target spectral line width (F2 feeding subroutine 3).

With the present invention, the halogen gas feeding amount is determinedin accordance with the spectral line width, which is in proportionalrelationship with the number of molecules of halogen gas. That is, whencontrol is performed to make the spectral line width the prescribedtarget value, as halogen gas is consumed, the spectral line widthbecomes narrower, so the feed amount of halogen gas is increased in thiscase such as to make the spectral line width broader. And if thespectral line width has become broader, the feed amount of halogen gasis reduced so as to make the spectral line width narrower.

Thus, with the ninth invention, the oscillation spectral line width,which is in proportional relationship with the halogen gas partialpressure is detected, and the feeding of amount of halogen gas isdetermined in accordance with the difference of this detected line widthvalue and the target spectral line width; an accurate quantity ofhalogen gas can therefore be supplied maintaining the target spectralline width.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram employed in the first, second and thirdembodiments of the present invention;

FIG. 2 is a main routine flow chart illustrating the overall operationof the first embodiment;

FIG. 3 is a flow chart showing power correction subroutine 1;

FIG. 4 is a flow chart showing Kr/Ne mixed gas feeding subroutine 1;

FIG. 5 is a flow chart illustrating F2 feeding subroutine 1;

FIG. 6 is a flow chart showing a charging voltage instructionsubroutine;

FIG. 7 is a flow chart showing a gas exchange request signal outputsubroutine;

FIG. 8 is a main routine flow chart showing a second embodiment of thepresent invention;

FIG. 9 is a flow chart showing power correction subroutine 2;

FIG. 10 is a flow chart showing power correction subroutine 3;

FIG. 11 is a main routine flow chart showing a third embodiment of thepresent invention;

FIG. 12 is a flow chart showing a Kr/Ne mixed gas feeding subroutine 2;

FIG. 13 is a block diagram showing a fourth embodiment of thisinvention;

FIG. 14 is a main routine flow chart showing a fourth embodiment of thepresent invention;

FIG. 15 is a flow chart showing a Kr/Ne mixed gas feeding subroutine 3;

FIG. 16 is a block diagram showing a fifth embodiment of this invention;

FIG. 17 is a main routine flow chart showing a fifth embodiment of thepresent invention;

FIG. 18 is a flow chart showing F2 feeding subroutine 2;

FIG. 19 is a block diagram showing a sixth embodiment of this invention;

FIG. 20 is a main routine flow chart showing a sixth embodiment of thepresent invention;

FIG. 21 is a block diagram showing a seventh embodiment of thisinvention;

FIG. 22 is a graph showing the experimental results regarding therelationship between F2 concentration and spectral line width;

FIG. 23 is a graph showing experimental results regarding therelationship between F2 partial pressure and spectral line width;

FIG. 24 is a main routine flow chart showing a seventh embodiment of thepresent invention;

FIG. 25 is a flow chart showing F2 feeding subroutine 3;

FIG. 26 is a block diagram showing an eighth embodiment of thisinvention;

FIG. 27 is a main routine flow chart showing an eighth embodiment of thepresent invention;

FIG. 28 is a flow chart showing F2 feeding subroutine 4;

FIG. 29 is a view showing a prior art gas supplementation controllayout;

FIG. 30 is a flow chart showing prior art gas supplementation control;and

FIGS. 31(a) to 31(d) are graphs showing the amounts of variousconditions produced in the prior art gas supplementation control.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention is described hereinbelow in detail with referenceto embodiments illustrated in the drawings.

When operation of an excimer laser apparatus is continued, with suchcontinued operation, the laser output falls off, due to consumption ofthe halogen gas and generation of impurities. The laser output isobtained by applying electrical energy stored on a capacitor forexcitation of laser gas to a discharge space, thereby discharging itthrough the laser medium gas.

The following three techniques are available as general means forcompensating for the drop in laser output:

(1) Raising the charging voltage V to the capacitor;

(2) Feeding Kr/Ne mixed gas to raise the total pressure;

(3) Compensating for the decrease in F2 gas by feeding of F2/Ne mixedgas.

First embodiment

A first embodiment of the present invention is described below withreference to FIG. 1 to FIG. 7.

FIG. 1 shows the layout of a gas feeding system for a KrF excimer laser;feeding of gas into laser chamber 1 is achieved by means of aconstruction comprising: a cylinder 2 charged with halogen gas (in thiscase fluorine) F2 diluted with Ne as buffer gas, a cylinder 3 chargedwith buffer gas Ne and diluent gas (in this case krypton) Kr in anarbitrary mixing ratio, and gas feeding on/off valves 4, 5 arranged onthe supply path of the gases from the cylinders to laser chamber 1.

Discharge of gas from laser chamber 1 is effected by means of aconstruction comprising a gas discharge on/off valve 6, and a vacuumcylinder 7.

It will be assumed that cylinder 2 contains F2 and Ne in concentrationratio F2:Ne=5:95 (%), while cylinder 3 contains Kr and Ne inconcentration ratio Kr:Ne=1.5:98.5 (%).

Oscillator 8 is an oscillator for internal triggering for producingpulsed oscillation; an oscillation instruction signal constituted by apulse signal is output from this oscillator 8 to discharge power source9 and a signal S synchronized with this oscillation instruction pulsesignal is output to a controller 10.

Discharge power source 9 serves to effect discharge between twoelectrodes within the laser chamber 1 at a frequency corresponding tothe oscillation instruction pulse signal that is output from oscillator8, and is temporarily charged up by a charging circuit with a voltagecorresponding to the instruction charging voltage V that is input fromcontroller 10; it then performs discharge by operation of a switchelement such as a thyratron. When the laser gas is excited by performingdischarge within laser chamber 1, laser oscillation is produced by anoptical oscillator, not shown, resulting in output of laser light. Itshould be noted that the discharge is implemented as a pulsedoscillation.

Output monitor 11 detects the energy E (hereinbelow termed the laseroutput) of laser light that is output, and also detects the fact thatlaser light is being output; its detected value E is input to controller10.

Fluorine concentration monitor 12 detects the fluorine concentration Fin laser chamber 1 and inputs the detected value F to controller 10.

Pressure monitor 13 detects the total pressure P of the gas in laserchamber 1; detected value P is input to controller 10.

Controller 10 uses the output E of output monitor 11 to calculate aninstruction charging voltage V, in a way to be described; this is outputto discharge power source 9, thereby controlling the discharge voltage.Also, controller 10 exercises control by controlling opening and closingof valves 4 and 5 during gas exchange or gas feeding during or prior tolaser operation, such that the amount of gas fed into laser chamber 1 isa prescribed amount. Also, controller 10 calculates the partial pressureFp of the fluorine from the fluorine concentration F and total gaspressure P. Furthermore, controller 10 monitors the power lock chargingvoltage (charging voltage when the laser output E has attained itsprescribed value) Vp etc and is equipped with a memory table that storesthese monitored values.

The operation of a first embodiment, performed chiefly by controller 10,will be described below with reference to the flow charts of FIG. 2 toFIG. 7.

First of all, the various parameter values listed below are set torespective prescribed values (step 100a).

Tt: lower limit time for output correction . . . lower limit time foroutput correction on stoppage of oscillation

If the period of oscillation stoppage is long, the laser output islowered, so if the period of oscillation stoppage exceeds time Tt, thedrop in laser output is compensated by feeding of Kr/Ne mixed gas.

Pa: total pressure on termination of feeding . . . upper limit of totalpressure on gas feeding

When the total pressure exceeds Pa, feeding of Kr/Ne mixed gas isstopped.

Vc: Kr/Ne mixed gas feeding charging voltage . . . threshold value ofcharging voltage at which feeding of Kr/Ne mixed gas is to be performed.

Tg: Kr/Ne mixed gas feeding interval time

If, after the previous occasion of feeding of Kr/Ne mixed gas, the timeat which laser oscillation was achieved after the charging voltage hadreached Vc exceeds Tg, a prescribed amount of Kr/Ne mixed gas is fed.

Fc: target partial pressure of fluorine

ΔFc: allowed width of target partial pressure of fluorine (±ΔFc)

Ec: rated laser output

ΔVc: minimum amount of increase/decrease of charging voltage

Va: upper limit of charging voltage

Sc: target oscillation shot number for one feed gas

Dc: target feeding time for one feed gas

Gn: Kr/Ne mixed gas feeding amount

Also, the variables listed below are not initially set in step 100a, butare employed in the processing to be described below, so they aredefined below.

ΔVm: allowed width of charging voltage (±ΔVm)

St: number of shots of interval of feeding of Kr/Ne mixed gas

Sn: number of shots of interval of F2 feeding

λc: target spectral line width

Δλc: allowed width of target spectral line width (±Δλc)

Dt: feeding time of presently fed gas

St: number of oscillation shots of presently fed gas

Vp: power lock charging voltage

Sn: number of shots after feeding of Kr/Ne mixed gas on previousoccasion

Fp: partial pressure of F2

ΔFp: Fp-Fc

Pb: threshold value for total gas pressure for maintenance request.

When setting of the above parameters has been completed, the procedureshifts to the power correction subroutine 1 shown in FIG. 3 (step 101a).

This power correction subroutine 1 is a routine to compensate the laseroutput drop produced by gas exchange and a prolonged period ofcessation.

First of all, controller 10 checks for the presence of a gas exchangerequest signal (step 120); if there is an exchange request, it readsfrom the memory table the initial power lock voltage Vp of the gaspresently being fed (step 129) and compares this power lock chargingvoltage Vp with Kr/Ne mixed gas feeding charging voltage Vc (step 130).If, as a result of this comparison, it finds that Vp≦Vc, it performs gasexchange with the same gas composition as the gas exchange on thepreceding occasion (step 133). However, if Vp>Vc, it performs gasexchange with a gas composition wherein the pressure of feeding of theKr/Ne mixed gas is altered in accordance with ΔV (=Vp-Vc) (steps 132,133). That is, if the value of ΔV gets large, gas exchange is effectedwith a feeding pressure of Kr/Ne mixed gas that is proportionallylarger.

Specifically by the above control, it is arranged that, if Vp duringlaser oscillation on the previous occasion was higher than Vc, gasexchange is performed such that Kr/Ne mixed gas is added in a quantitycorresponding to ΔV; the prescribed laser output can therefore beobtained by a low power lock charging voltage in laser oscillation aftergas exchange.

Also, if the decision made in step 120 does not request gas exchange,controller 10 first of all calculates oscillation stop time T. It thencompares this oscillation stop time T with the output correction lowerlimit time Tt (step 123); if T≦Tt, the procedure returns to the originalmain routine; if T>Tt, an amount Gn of mixed Kr/Ne gas is calculatedcorresponding to the oscillation stop time T (step 124).

Next, controller 10 reads total pressure Pp prior to oscillationstoppage from the memory table (step 125) and calculates the totalpressure Pt after feeding from this calculated feeding amount of mixedKr/Ne gas Gn and the total pressure Pp prior to oscillation stoppage(step 126) and compares this calculated value Pt with the total pressurePa after completion of feeding. Then, if the calculated value Pt exceedsthe total pressure Pa on completion of feeding, the procedure returns tothe main routine without any action being taken; if the calculated valuePt does not exceed the total pressure Pa on completion of feeding, anamount Gn of mixed Kr/Ne gas is fed (steps 127, 128).

That is, with the above control, an amount of Kr/Ne mixed gascorresponding to the oscillation stop time is fed prior to laseroscillation, so the drop in laser output due to natural deterioration ofthe gas during oscillation stoppage can be forestalled.

It may be noted that, in the gas exchange performed in step 133 and thefeeding of mixed Kr/Ne gas performed in step 128, feeding of the desiredquantity of gas is performed by controlling the time for which valve 4or 5 is opened. In this gas feeding, the total pressure P in the laserchamber 1 is monitored by pressure monitor 13, and this detected value Pis used to control opening/closing of valve 4 or 5 such that thepressure within laser chamber 1 becomes a prescribed value. Gasdischarge during gas exchange is achieved by starting vacuum pump 7 andopening valve 6.

When power correction subroutine 1 has been completed as describedabove, laser oscillation is commenced by the main routine (FIG. 2 step102). During this laser oscillation, laser output E, charging voltage V,total pressure P, and fluorine concentration F respectively detected byoutput monitor 11, charging power source 9, pressure monitor 13 andfluorine concentration monitor 12 are input to controller 10 (step 103).

Next, the procedure shifts to the Kr/Ne mixed gas feeding subroutine 1shown in FIG. 4 (step 104a). This Kr/Ne mixed gas feeding subroutine 1is a routine for compensating the output that falls with lapsed time oflaser oscillation.

First of all, the charging voltage V is compared with the Kr/Ne mixedgas feeding charging voltage Vc (step 140); if V>Vc, the time Tv iscalculated at which laser oscillation occurred with a charging voltageof Vc or more after the feeding of Kr/Ne mixed gas on the previousoccasion (step 141). This time Tv is then compared with the Kr/Ne mixedgas feeding interval time Tg (step 142), and, if Tv>Tg, next, the totalpressure P is compared with the total pressure Pa after completion offeeding (step 143). If the result of this comparison is that P<Pa, aftera fixed quantity of Kr/Ne mixed gas has been fed (step 144), theprocedure returns to the main routine. It should be noted that, in thecomparison of step 140, if V≦Vc or, in the comparison of step 142, ifTv≦Tg, or, in the comparison of step 143, if P≧Pa, no action is taken asthe procedure returns to the main routine.

Furthermore, in the above step 144, a fixed quantity of Kr/Ne mixed gasis fed by opening valve 5 for a fixed time.

Thus, with the Kr/Ne mixed gas feeding subroutine as described above, ifthe charging voltage V exceeds a prescribed threshold value Vc, aprescribed amount of Kr/Ne mixed gas is fed corresponding to thecondition in which the charging voltage becomes equal to or greater thanthe aforesaid threshold value over a prescribed time interval (Tg), solaser output can be subjected to a fixed control that is reliable andhighly accurate, due to the feeding of rare gas/buffer gas in fixedamount in the gas supply control.

In this way, when the processing of the Kr/Ne mixed gas feedingsubroutine 1 is completed, next, the fluorine partial pressure Fp iscalculated from the F2 concentration F and total pressure P (step 105 inFIG. 2). The difference ΔFp between this F2 partial pressure Fp and thetarget F2 partial pressure Fc is then calculated (step 106). Thisdifference ΔFp is then compared with the allowed width ΔFc of the targetF2 partial pressure (step 107), and if |ΔFp|≧ΔFc, the procedure advancesto the F2 feeding subroutine 1 shown in FIG. 5 (step 108a), while if|ΔFp|<ΔFc, the procedure enters the charging voltage instructionsubroutine shown in FIG. 6 (step 109).

In the F2 feeding subroutine 1 of FIG. 5, total pressure P and totalpressure Pa on completion of feeding are compared (step 150); if P≧Pa,the procedure returns to the main routine; if P<Pa, a feeding amount Gtis calculated in accordance with ΔFp that was previously calculated(step 151); F2 gas in the amount of this calculated feeding amount Gt isthen fed and the procedure returns to the main routine (step 152).

In this way, in the F2 gas feeding subroutine 1, the F2 gas partialpressure Fp is detected and a halogen gas supplementation amountcorresponding to this detected partial pressure value Fp is determined;halogen gas can therefore be supplied precisely in the amount of theactual diminution of the halogen gas.

It should be noted that, although, in this case, a quantity of F2 gascorresponding to ΔFp was fed, it would be possible to arrange to feedalways a prescribed quantity of F2 gas, irrespective of ΔFp. Also,although, in this embodiment, the F2 partial pressure was controlled toa fixed value, it would be possible to control the F2 concentration to afixed value.

Thus, when the F2 feeding subroutine 1 is completed, the procedureshifts to the charging voltage instruction subroutine.

The charging voltage instruction subroutine shown in FIG. 6 is a routinefor setting charging voltage V corresponding to laser output E. In thischarging voltage instruction subroutine, the detected laser output E iscompared with the rated laser output Ec (step 160); if E<Ec, anincrement/decrement ΔVc is added to charging voltage V (step 161) suchas to raise laser output to the rated output, and the result of thisaddition (V+ΔVc) is output to discharge power source 3 as instructioncharging voltage V (step 163). Also, if the result of the comparisonperformed in step 160 is that E=Ec, the detected charging voltage V isoutput to discharge power source 3 as instruction charging voltage (step163). Furthermore, if the result of the comparison of step 160 is thatE>Ec, an increment/decrement ΔVc is subtracted from charging voltage Vsuch as to make the laser output fall to the rated output (step 161);the result of this subtraction (V-ΔVc) is output to discharge powersource 3 as instruction charging voltage V (step 163).

When the charging voltage instruction subroutine has been completed asabove, charging voltage V is compared with charging voltage upper limitvalue Va (step 110); if the result is that V≦Va, the procedure againshifts to step 103 and the same processing as described above isperformed; however, if the result of the comparison is that V>Va, theprocedure advances to the gas exchange request signal output subroutineshown in FIG. 7.

This gas exchange request signal output subroutine is a subroutine foroutputting a gas exchange request signal, a window exchange requestsignal of laser chamber 1, and a maintenance request signal.

In the gas exchange request signal output subroutine shown in FIG. 7,first of all, a gas exchange request signal is output from controller 10(step 170), and the operator is thereby alerted to the fact that theinstruction charging voltage V has got outside the allowed range, and adisplay or warning tone prompting gas exchange is issued.

Next, controller 10 calculates the oscillation shot number St of thepresently charged gas, and the gas charging time (number of days) Dt(step 171). This number of oscillation shots St is then compared withthe lower limit value Sc of oscillation shots for a single gas charge(step 172); if St≧Sc, the procedure returns to the main routine; ifSt<Sc, the previously found gas charging time Dt is compared with thelower limit value of the charging time for a single gas charge (step173). If the result of this comparison is that Dt≧Dc, the procedurereturns to the main routine; if Dt<Dc, next, the gas total pressure P iscompared with the threshold value Pb (step 174). Then, if P<Pb, theprocedure returns to the main routine; if P≧Pb, it is ascertainedwhether or not the exchange of gas on the current occasion is the firsttime the gas has been exchanged after window exchange (in other words,whether the present gas is the gas that was first introduced afterwindow exchange) (step 175). Then, if this is not the first gas change,a window exchange request signal is output to perform a warning actionto that effect; when, in response to this, the window is exchanged, theprocedure returns to the main routine (step 177).

Also, if this in fact the first gas change, a maintenance request signalis output to warn the operator to that effect so thatmaintenance/inspection can be carried out, after which the procedurereturns to the main routine (step 176).

Thus, with this first embodiment, before laser oscillation, the drop inlaser output produced by a protracted period of stoppage is compensatedfor by feeding Kr/Ne mixed gas in an amount corresponding to the periodT of oscillation stoppage. Also, after laser oscillation, laser outputcompensation control is performed whereby Kr/Ne mixed gas is fed in anamount responsive to the charging voltage and the F2 gas is supplementedin an amount responsive to the partial pressure value of the F2 gas,and, furthermore, the charging voltage is increased or decreased inresponse to the result of a comparison of laser output with rated laseroutput until the charging voltage rises up to a prescribed upperlimiting value.

Second embodiment

Next, a second embodiment, which is a modification of the firstembodiment, will be described with reference to the flow charts of FIG.8 to FIG. 10. This second embodiment is the same as the first embodimentexcept for the fact that the power correction subroutine 1 of the mainroutine of FIG. 2 of the first embodiment described above is changed topower correction subroutine 2 (step 101b in FIG. 8), and that a powercorrection subroutine 3 (step 101c) is added as a power correction afterlaser oscillation. Description which would be duplicated is omitted.

Power correction subroutine 2 shown in FIG. 9 omits step 121 to step 128of power correction subroutine 1 of the first embodiment above.

In more detail, first of all, controller 10 checks to see whether thereis a gas exchange request signal (step 120 in FIG. 9); if there is anexchange request, it reads (step 129) from the memory table the initialpower lock voltage Vp of the presently charged gas, and compares thispower lock charging voltage Vp with the Kr/Ne mixed gas feeding chargingvoltage Vc (step 130). If the result of this comparison is that Vp≦Vc,gas exchange is performed with the same gas composition as in the gasexchange performed on the previous occasion (step 133). However, ifVp>Vc, gas exchange is performed (steps 132, 133) with an alteredcharging pressure of the Kr/Ne mixed gas, corresponding to ΔV (=Vp-Vc).

Also, if, in step 120, there is no request for gas exchange, controller10 returns to the main routine.

The power correction subroutine 3 shown in FIG. 10 is a procedure thatis performed after laser excitation, in order to compensate for the dropin laser output produced by prolonged laser stoppage and is a substituteprocedure for step 121 to step 128 of power correction subroutine 1 ofthe first embodiment, which was omitted in power correction subroutine 2above.

In this power correction subroutine 3, the charging voltage 3 when laseroutput E has reached rated output Ec is detected (steps 180 to 182),and, if V>Vc, a feeding amount Gn of Kr/Ne mixed gas corresponding tothis charging voltage is calculated (step 183), and this calculatedamount Gn of Kr/Ne mixed gas is fed, after which the procedure returnsto the main routine. That is, in this second embodiment, even when thelaser has been stopped for a long time, laser oscillation is commencedwithout feeding of Kr/Ne gas and then subsequently an amount of Kr/Nemixed gas corresponding to the charging voltage value after laseroscillation is fed at once. Consequently, even if, because of the longperiod of stoppage, the laser was in a condition in which the chargingvoltage V was higher than at first, it is possible to feed all at once alarge quantity of rare gas and buffer gas corresponding to this largecharging voltage value; thus, the charging voltage value can be loweredall at once, compared with the conventional system, in which thequantity of gas feeding is fixed.

It should be noted that, in this second embodiment, just as in the firstembodiment above, the feeding amount Gn of Kr/Ne mixed gas may becalculated from the time T of oscillation stoppage. Also, since, ifseveral Torr of Kr/Ne mixed gas are fed at one time during laseroscillation this will produce considerable fluctuation of laser output,it is preferable to feed Kr/Ne mixed gas feeding amounts Gn above 3 Torrin several separate smaller amounts.

Third embodiment

A third embodiment of the present invention is described below withreference to FIG. 11 to FIG. 12.

In this third embodiment, the Kr/Ne mixed gas feeding subroutine 1 (step104a of FIG. 2) of the first embodiment above is replaced by the Kr/Nemixed gas feeding subroutine 2 (step 104b in FIG. 11) shown in FIG. 12.Also, the Kr/Ne mixed gas feeding amount Gn is added to the initiallyset parameters in step 100b. Apart from this, it is exactly the same asthe first embodiment.

Specifically, in the Kr/Ne mixed gas feeding subroutine 2 of FIG. 12,the laser shot number Sn after feeding of Kr/Ne mixed gas is calculated(step 190) and this shot number Sn and the shot number St (fixed value)of the interval in which Kr/Ne mixed gas is fed are compared (step 191);if Sn≦St, the procedure returns to the main routine, while if Sn>St,Kr/Ne mixture is fed in a fixed amount Gn that was initially set; theprocedure then returns to the main routine (step 192).

Specifically, in this Kr/Ne mixed gas feeding subroutine 2, the numberof laser shots after the feeding of Kr/Ne mixed gas on the previousoccasion is calculated, and if this shot number exceeds a prescribedthreshold value, mixed gas is fed; the feeding of rare gas is thereforeperformed in a manner which is based on the number of laser shots, whichis directly related to the rate of laser oscillation; constant controlof laser output with precision and high accuracy can thereby beachieved.

Fourth embodiment

A fourth embodiment of the present invention will now be described withreference to FIG. 13 to FIG. 15.

In the fourth embodiment shown in FIG. 13, a mass flow controller (massflow control device) 14 and on/off valve 15 for the Kr/Ne mixed gassupply are added to the construction of FIG. 1 above.

Specifically, if several Torr of Kr/Ne mixed gas are fed at once, thelaser output fluctuates. The amount of this fluctuation becomes less asthe amount of feeding on a single occasion is decreased. With feedingusing an on/off valve 4, the minimum that can be fed is about 1 Torr,but if a mass flow controller 14 is employed, more minute amounts offeeding are possible enabling fluctuation of laser output to besuppressed. This is the reason for employing a mass flow controller 14.

FIG. 14 shows the main routine of this fourth embodiment. In this fourthembodiment, the Kr/Ne mixed gas feeding subroutine 1 of the firstembodiment is replaced by the Kr/Ne mixed gas feeding subroutine 3 (step104c of FIG. 14) shown in FIG. 15. Also, the allowed width ΔVm of thecharging voltage is added to the initially set parameters in step 100c.Aside from this, this embodiment is the same as the first embodimentabove.

In the Kr/Ne mixed gas feeding subroutine 3 of FIG. 15, the differenceΔV (=V-Vc) of the charging voltage V and Kr/Ne mixed gas feedingcharging voltage Vc is found (step 200), and this difference ΔV iscompared with the permitted width ΔVm of the charging voltage (step201). If the result of this comparison is that |ΔV|>ΔVm, the flow rateof Kr/Ne mixed gas of the mass flow controller is altered (step 202) andthe procedure then returns to the main routine; if the result is that|ΔV|≦ΔVm, no action is taken and the procedure returns to the mainroutine.

It should be noted that, although in the embodiments described above,on/off valve 15 was always open, if the flow rate of mass flowcontroller 14 is made constant, on/off valve 15 can be on/off controlledin accordance with ΔV.

Fifth embodiment

A fifth embodiment of the present invention will be described withreference to FIG. 16 to FIG. 18.

In the fifth embodiment shown in FIG. 16, fluctuation of laser outputdue to fluorine gas feeding is prevented by adding a mass flowcontroller 16 and on/off valve 17 for supply of F2 gas to theconstruction of FIG. 1 above.

Specifically, in this fifth embodiment, the amount of gas passingthrough a gas supplementation path is controlled so that the mass flowrate of halogen gas that is supplied by mass flow controller 16 is thedesired fixed value.

FIG. 17 shows the main routine according to this fifth embodiment; theF2 gas feeding subroutine 1 of the first embodiment above is replaced bythe F2 gas feeding subroutine 2 (step 108b in FIG. 17) shown in FIG. 18.Otherwise, this embodiment is exactly the same as the first embodimentabove.

In the F2 gas feeding subroutine 2 shown in FIG. 18, the flow rate offluorine gas supplied at mass flow controller 16 is altered inaccordance with ΔFp (=Fp-Fc).

It should be noted that, although in the embodiment described aboveon/off valve 17 is always open, it could be arranged to keep the flowrate of mass flow controller 16 constant and to perform on/off controlof on/off valve 17 in response to ΔFp.

Sixth embodiment

In the sixth embodiment shown in FIG. 19, neither the fluorineconcentration monitor 12 of FIG. 1 nor the spectral width monitor 20adopted in subsequent embodiments are provided. That is, in this sixthembodiment, insufficiency of F2 is inferred from the laser shot numberafter feeding of fluorine gas and if the F2 is judged to beinsufficient, a previously set fixed quantity of fluorine is supplied.

FIG. 20 shows the main routine of this sixth embodiment; in thisembodiment, the fluorine feeding processing procedure (step 105 to step108a) of the first embodiment is replaced by the procedure of step 220to step 222). Also, in the initial setting of parameters of step 100d,the feeding amount Gt of F2 gas is set. Otherwise this embodiment isexactly the same as the first embodiment.

Specifically, when the Kr/Ne mixed gas feeding subroutine 1 terminates(step 104a), the number of shots St since feeding of fluorine gas on theprevious occasion is calculated (step 220), and this calculated value Stand the number of shots Sh in the F2 feeding interval are compared (step221). Then, if St≧Sh, the previously set fixed quantity Gt of F2 gas isfed (step 222); if St<Sh, the following step is executed.

Seventh embodiment.

A seventh embodiment is described with reference to FIG. 21 to FIG. 25.

In this seventh embodiment, in place of the fluorine concentrationmonitor, a spectral line width monitor 20 is provided; this embodimentis applied to lasers of narrow bandwidth.

Specifically, in a narrow-bandwidth laser, the spectral line width mustbe kept below a prescribed value, but, as the spectral line widthbecomes smaller, the oscillation efficiency of the laser decreases. Itis therefore effective to have the laser oscillate with its spectralline width controlled such as to be in the neighborhood of a prescribedvalue.

In this connection, noting that the spectral line width λ has a closecorrelation with the fluorine partial pressure Fp, the present inventorsarranged to determine the halogen feeding amount in accordance with thespectral line width λ, which is in inverse proportional relationshipwith the number of molecules of halogen gas. Specifically, if thespectral line width is controlled such as to be a prescribed targetvalue, the spectral line width becomes narrower as halogen gas isconsumed, so, in this case, the halogen gas supply rate is increasedsuch as to make the spectral line width broader. Also, when the spectralline width has broadened, the halogen gas supply rate is diminished soas to make the spectral line width narrower.

FIG. 22 shows experimental results expressing the relationship betweenF2 concentration and spectral line width; FIG. 23 shows experimentalresults expressing the relationship between F2 partial pressure andspectral line width. Looking at these graphs, it can be seen that thereis no correlation between the F2 concentration and spectral line widthshown in FIG. 22, but there is a clear inverse proportional relationshipbetween the F2 partial pressure and spectral line width shown in FIG.23.

From these experimental results, the present inventors concluded that"line width is proportional to number of F2 molecules (proportional toF2 partial pressure). However, since the percentage concentration in themixed gas changes with increase or decrease of other gases, this doesnot reflect accurately the molecules i.e. the spectral line width."

This can also be proved from the state equation of the gas.

If now we let the partial pressure of each gas in the mixed gas be Pi(i=˜n), the number of molecules of each gas in the mixed gas be Ni(i=˜n), the internal volume of the laser chamber be v, the gas constantbe R, and the gas temperature t, the following equation is established:

    ΣPi·v=ΣNi·R·t       (1)

If now we imagine that the number of molecules is increased to Nj+δn byadding a certain gas, from the above equation (1), the followingequation (2) can be obtained:

    (P1+P2+ . . . +Pj . . . +Pi+δP)×v=(N1+N2+ . . . +Nj . . . +Ni+δN)×R×t                             (2)

By subtracting equation (1) from equation (2), the following equation(3) can be obtained:

    δP×v=δN×R×t                  (3)

Since v, R and t are all constants, on focusing on the individual gasesin the mixed gas, it can be seen that the increase in number ofmolecules of a gas and increase in partial pressure are proportional, sothe amount of increase of oscillation line width produced by supply ofgas can be quantitatively predicted.

FIG. 24 shows the main routine in this seventh embodiment; in thisembodiment, the fluorine feeding processing procedure of the firstexample (step 105 to step 108a) is substituted by the processing of step230, step 231, and step 108c. Also, in the initial setting of parametersin step 100e, the target spectral line width λc and the permitted widthΔλc of the spectral line width are arranged to be set. Otherwise, thisembodiment is exactly the same.

In more detail, when laser oscillation starts (step 102), laser outputE, charging voltage V, total pressure P, and spectral line width λrespectively detected by output monitor 11, discharge power source 9,pressure monitor 13, and spectral line width monitor 20 during laseroscillation are input to controller 10 (step 103).

After this, when the Kr/Ne mixed gas feeding subroutine 1 has terminated(step 104), the difference Δλ between the detected spectral line width λand the target spectral line width λc is calculated (step 230), and thisdifference Δλ is compared with the permitted width Δλc of the line width(step 231). If, in this comparison, |Δλ|≧Δλc, the procedure advances tothe F2 feeding subroutine 3 shown in FIG. 25 (step 108c); if |Δλ|<Δλc,the procedure advances to the next step, and the charging voltageinstruction subroutine is executed.

In F2 feeding subroutine 3 of FIG. 25, total pressure P is compared withthe total pressure Pa after completion of feeding (step 240); if P<Pa,the previously calculated Δλ is used to calculate the F2 gas feedingquantity (step 241), and a quantity of F2 gas equal to this calculatedquantity is fed (step 242). In more detail, Δλ and feeding amount Gt arein approximately inverse proportional relationship, so, if line width λis smaller than the target value, the amount of halogen gas supplied isincreased so as to make the spectral line width broader, while, if thespectral line width is broader than the target value, the amount ofhalogen gas supplied is decreased so as to make the spectral line widthnarrower.

If, in the comparison of step 240 above, P≧Pa, the procedure returns tothe main routine.

The subsequent processing is the same as in the case of the firstembodiment above.

Eighth embodiment

An eighth embodiment will-now be described with reference to FIG. 26 toFIG. 28.

In this eighth embodiment, as shown in FIG. 26, just as in the case ofthe seventh embodiment above, there is provided a spectral line widthmonitor 20, and a mass flow controller 16 is employed for supplying theF2 gas.

FIG. 27 shows the main routine in this eighth embodiment; in this eighthembodiment, after commencement of laser oscillation (step 102), controlof feeding of F2 gas is performed in accordance with a line widthcomparison and (steps 229 to 231, 108c), after Kr/Ne mixed gas feedingsubroutine 1 (step 104a), F2 gas feeding control is again exercisedbased on a line width comparison. Otherwise, this embodiment is the sameas the seventh embodiment.

Specifically, laser oscillation is commenced (step 102); spectral linewidth λ is detected (step 229), the difference Δλ between this detectedspectral line width λ and the target spectral line width λc iscalculated (step 230); and this difference Δλ is compared with thepermitted width Δλc of the line width (step 231). If, in thiscomparison, |Δλ|≧Δλc, the procedure advances to the F2 feedingsubroutine 3 shown in FIG. 25 above (step 108c); if |Δλ|<Δλc, theprocedure advances to the next step 103.

In the F2 feeding subroutine 3 of FIG. 25, total pressure P is comparedwith the total pressure Pa after completion of feeding (step 240); ifP<Pa, the previously calculated Δλ is used to calculate the F2 gasfeeding quantity (step 241); and an amount of F2 gas equal to thiscalculated quantity is fed (step 242).

Also, when Kr/Ne mixed gas feeding subroutine 1 has been completed (step104a), the difference Δλ between the detected spectral line width λ andthe target spectral line width λc is again calculated (step 250), andthis difference Δλ is compared with the permitted width Δλc of the linewidth (step 251). If in this comparison, |Δλ|≧Δλc, the procedureadvances to the F2 feeding subroutine 4 shown in FIG. 28 (step 108d); if|Δλ|<Δλc, the procedure advances to the next step and the chargingvoltage instruction subroutine is executed (step 109).

In F2 feeding subroutine 4 of FIG. 28, the F2 gas flow amount passingthrough mass flow controller 16 is altered in accordance with Δλ (step260).

It should be noted that, although, in the embodiment described above,on/off valve 17 was always open, it would be possible to arrange to havethe flow amount of the mass controller 16 fixed and to effect on/offcontrol of on/off valve 17 in response to Δλ.

Also, in the present invention, He or Ne and He mixed gas can be used asbuffer gas, HCl can be used as halogen gas, and Xe or Ar can be used asrare gas.

Also, although, in the embodiments, the Kr/Ne mixed gas was stored in asingle cylinder, these could be stored in separate cylinders and controlperformed such that the prescribed mixing ratio is achieved within thelaser chamber.

Furthermore, although, in the main routines illustrated in FIG. 2, FIG.8, FIG. 11, FIG. 14, FIG. 17, FIG. 20, FIG. 24 and FIG. 27, V and Vawere compared and oscillation was stopped if V>Va, in the case of theexposure period, the laser could be allowed to continue oscillation, theprocedure shifting to the gas exchange request signal output subroutineon input of oscillation stoppage permission.

INDUSTRIAL APPLICABILITY

The present invention can be utilized in an excimer laser apparatusemployed in a light source for size-reduction projection exposure,microprocessing of materials, and surface improvement of materials etc.

What is claimed is:
 1. A method of gas supplementation of an excimerlaser apparatus wherein laser oscillation is performed with feeding ofhalogen gas, rare gas, and buffer gas into a laser chamber,characterized in that:before laser oscillation, an oscillation stop timeis calculated, and, if the calculated oscillation stop time exceeds aprescribed time, the calculated oscillation stop time is used tocalculate a feeding amount of mixed gas comprising rare gas and buffergas, and the mixed gas is fed, prior to laser oscillation, in thecalculated feeding amount.
 2. A method of gas supplementation of anexcimer laser apparatus according to claim 1, wherein a predictivecalculation is made of a total pressure after feeding of the mixed gas,and, only if the calculated total pressure does not satisfy a prescribedvalue, the mixed gas in the calculated feeding amount is fed.
 3. Amethod of gas supplementation of an excimer laser apparatus whereinlaser oscillation is performed with feeding of halogen gas, rare gas andbuffer gas into a laser chamber, characterized in that:during laseroperation, a halogen gas partial pressure in the laser chamber isdetected, and a halogen gas supplementation amount is determinedcorresponding to the detected partial pressure value.
 4. A method of gassupplementation of an excimer laser apparatus according to claim 3,characterized in that:a difference is found between the detected partialpressure value of the halogen gas and a target halogen gas partialpressure, and a supplementary amount of halogen gas that is supplied isdetermined in accordance with the difference.
 5. A method of gassupplementation of an excimer laser apparatus according to claim 4,characterized in that:supplementation of the halogen gas is performedwhen the difference exceeds a prescribed allowable width and a totalpressure within the laser chamber does not satisfy the prescribed value.6. A method of gas supplementation of an excimer laser apparatus whereinlaser oscillation is performed with feeding of halogen gas, rare gas andbuffer gas into a laser chamber, characterized in that:prior to laseroscillation, feeding of mixed gas comprising rare gas and buffer gas isnot performed, a laser output is detected immediately after laseroscillation, and if the detected laser output exceeds a predeterminedrated laser output, a charging voltage is detected and the mixed gascomprising rare gas and buffer gas is fed in the amount calculatedcorresponding to the detected charging voltage.
 7. A gas supplementationmethod of excimer laser apparatus in which laser oscillation isperformed with feeding of halogen gas, rare gas and buffer gas into alaser chamber, wherein:prior to laser oscillation, if a gas exchangerequest signal is output, an initial power lock voltage in a presentlycharged gas is detected and the detected value is compared with aprescribed threshold value and, if the detected value is smaller thanthe threshold value, gas exchange is effected with a gas composition thesame as on a previous occasion; but, if the detected value is largerthan the threshold value, the fed amount of mixed gas comprising raregas and buffer gas is altered in accordance with a deviation between thedetected value and the threshold value, and gas exchange is performedwith a gas composition based on the altered feeding amount.
 8. A methodof gas supplementation of an excimer laser apparatus according to claim7, wherein:the gas exchange request signal is output if a chargingvoltage exceeds an upper limit value of the charging voltage.
 9. A gassupplementation method of an excimer laser apparatus, in which laseroscillation is performed with feeding of halogen gas and rare gas, andbuffer gas into a laser chamber and a mixed gas comprising rare gas andbuffer gas is fed in a prescribed amount if a charging voltage exceeds aprescribed threshold value during laser oscillation, characterized inthat:if the charging voltage exceeds the prescribed threshold valueduring laser oscillation, after mixed gas comprising rare gas and buffergas was fed on the previous occasion, a time interval for whichoscillation occurred with the charging voltage in excess of thethreshold value is measured and at a time point where the time intervalexceeds a prescribed time, the mixed gas is fed in the prescribedamount.
 10. A method of gas supplementation of an excimer laserapparatus according to claim 9 wherein feeding of the mixed gas isperformed if a total gas pressure in the laser chamber does not satisfya prescribed threshold value gas pressure.
 11. A gas supplementationmethod of an excimer laser apparatus in which laser oscillation isperformed with feeding of halogen gas, rare gas and buffer gas into alaser chamber, characterized in that:if a gas exchange request signal isoutput prior to laser oscillation, a window exchange signal or amaintenance request signal is output in response to an oscillation shotnumber of a presently charged gas, a period of charging of the presentlycharged gas, a gas pressure within the laser chamber, or the number oftimes of gas charging after window exchange of the laser chamber.
 12. Amethod of gas supplementation of an excimer laser apparatus according toclaim 11, wherein the gas exchange request signal is output when acharging voltage exceeds un upper limit value of the charging voltage.13. In an excimer laser apparatus in which laser oscillation isperformed with feeding of halogen gas, rare gas, or buffer gas into alaser chamber, characterized in that a gas supplying devicecomprises:quantity flow rate controller for supplying to the laserchamber a mixed gas comprising rare gas and buffer gas supplied from agas bomb; charging voltage detecting means for detecting a chargingvoltage after laser oscillation; and control means for determining adifference between the detected charging voltage and an upper limit ofthe charging voltage and changing a supply flow rate of the quality flowrate controller in accordance with the determined differences.
 14. Anexcimer laser apparatus according to claim 13, wherein feeding of mixedgas is performed when the difference exceeds a prescribed allowablewidth.
 15. A method of gas supplementation of an excimer laser apparatusin which laser pulse oscillation is performed at a predeterminedrepetitive frequency with feeding of halogen gas, rare gas and buffergas into a laser chamber, characterized in that:a laser shot number fromthe feeding of mixed gas comprising rare gas and buffer gas on aprevious occasion is calculated and feeding of the mixed gas isperformed if the shot number exceeds a prescribed threshold value.
 16. Amethod of gas supplementation of an excimer laser apparatus in whichlaser oscillation is performed with feeding of halogen gas, rare gas andbuffer gas into a laser chamber, characterized in that:an oscillationspectral line width is detected and an amount of halogen gas feeding isdetermined in accordance with a difference between the detected linewidth value and a target spectral line width.
 17. A method of gassupplementation of an excimer laser apparatus according to claim 16,wherein:the feeding of the halogen gas is performed if a total gaspressure within the laser chamber does not satisfy a prescribedthreshold value gas pressure.